1. Field of the Invention
The present invention relates to the plating of aluminum and aluminum alloys, and, more particularly, to the plating of 390 aluminum alloys with iron.
2. Description of Related Art
The automotive industry is investigating the use of aluminum engines with aluminum pistons to reduce the weight of the vehicle and to meet fuel economy requirements. However, it is essential that either the piston or the cylinder bore be coated with a metal harder than aluminum to prevent piston skirt scuffing during cold starts. Therefore, an inexpensive, environmentally sound method of plating the piston with iron would be valuable.
Presently, an iron coating is plated onto the surface of the aluminum pistons, generally employing a copper undercoat. In one process, copper cyanide and iron chloride baths are used in the plating. Copper cyanide is a highly toxic and tightly regulated material. The iron chloride bath is also a highly toxic and extremely corrosive bath that is very destructive to the equipment around it.
An alternative approach is to insert an iron sleeve into the cylinder bore. Still another approach is to coat the inside of the bore with a suitable metal alloy by thermal spray coating processes and then re-machining the bore. These approaches are estimated to be 8 to 14 times as expensive as piston plating.
The previous methods employ many steps. Examples of the previous art for plating on aluminum may be found in Metal Finishing Guidebook and Directory Issue '92, pages 157-158, and Electroplating Engineering Handbook, Fourth Edition, edited by Larry Durney, pages 185-188.
In a recent patent application (application Serial No. 07/959,881, filed Oct. 13, 1992) filed in the name of the present inventor and assigned to the same assignee as the present application, a process of plating iron onto aluminum is disclosed and claimed, using a zincate process. However, while the teachings of that invention are certainly useful, it requires the added zincate step. Further, one cannot plate iron directly onto the zincate layer without an additional adhesion layer such as nickel or copper. The reason for this lies in the mechanism of the zincate itself. Zincate readily deposits an immersion layer on aluminum, but the reduction step to the next metal is limited to those metals that will reduce over the zincate without dissolving and destroying the very thin zinc coating. Also, zincate often requires a postplate bake to enhance the adhesion.
It is desired to provide a method, preferably inexpensive, for plating aluminum pistons with an acceptable iron coating that will pass all the required adhesion, hardness, and abrasion tests without using highly toxic or hazardous substances.